An example air-conditioning apparatus of this type is disclosed in JP-U-3-64108A, for example. The disclosed air-conditioning apparatus includes an air intake box having a door capable of pivotal movement to allow selective entry of inside or outside air, a duct in air communication with the box, a blower disposed between the box and the duct, a hot-water heater housed in the duct and designed to use engine-cooling water as a heat source, and a manual switch designed to be manually operated by a passenger in a passenger compartment to adjust a temperature of the compartment.
At a downstream end of the duct, there are formed discharge ports for discharging air from the duct while an electric heater is disposed adjacent to and downstream of the hot-water heater. The electric heater is located upstream of the downstream end of the duct and is supplied with electrical power from a source of electrical power. Disposed downstream of the electric heater is a temperature sensor for detecting a temperature of air passing through the electric heater.
The air-conditioning apparatus further includes a control unit for controlling the source of electrical power to supply electrical power to the electric heater when the following three conditions are satisfied: (1) a temperature of the air detected by the temperature sensor is lower than a temperature of the air obtained as the cooling water rises in temperature; (2) the fan is driven; and (3) the manual switch is adjusted to provide a maximum temperature of the passenger compartment.
In order that the air-conditioning apparatus provides an improved heating performance immediately after the engine is started, the electric heater keeps operating until the temperature of the cooling water reaches a predetermined temperature after the engine is started.
For the air-conditioning apparatus, however, defogging operation is less efficiently performed on a fogged window because a large amount of heated air to remove fog from the window can not be supplied to the window due to the presence of the electric heater acting as a resistance to a flow of the heated air flowing past the hot-water heater. Thus, the air-conditioning apparatus needs to be improved to provide a sufficient defogging performance as well as a heating performance.